1. Field of the Invention
The present invention relates to a powder containing activated coke and inert materials.
2. Background Information
Activated cokes, such as activated carbon on a basis of hard coal or peat, or metallurgical cokes on a basis of lignite, are sometimes used with the admixture of inert materials for cleaning exhaust gas. Such mixtures have been used to remove acid pollutants (e.g. sulfuric acid, hydrochloric acid, hydrofluoric acid), organic substances such as chlorinated hydrocarbons (e.g. dioxins and furans), as well as heavy metals and heavy metal compounds from exhaust gas. In installations which function according to the airstream adsorption principle, such means are distributed as uniformly as possible in the current of smoke or chimney gas, react in flight when airborne with gaseous pollutants and other pollutants, and are precipitated on filters, along with other particulate constituents of the exhaust gas. The cleaned exhaust gas is then released through a chimney.
For the precipitation of acid pollutant gases, such as HCl, SO2, SO3 and HF in the airstream process, reactive calcium compounds have been used in the past, such as finely-divided calcium hydroxide, Ca(OH)2. Calcium hydroxides for injection into the current of exhaust gases can be produced by various known processes, e.g. the process disclosed in German Patent DE 34 33 228.
Such known processes include the use of activated cokes, such as activated carbons and open hearth lignite coals, for the removal of heavy metals, heavy metal compounds and organic compounds such as dioxins and furans, e.g. those which are found in the exhaust gases from municipal waste incineration plants.
One disadvantage of such processes is that when handling combustible adsorbents, even if the rules of the art are observed, the use of activated cokes can entail the risk of dust explosions. In mixtures of inert materials with activated cokes, mixtures which are typically considered as non-explosive generally include at least mixtures with a concentration of carbon of less than or equal to 30% by weight. But, there are additional safety requirements which must typically be met for the use of these powdered means which include activated cokes and inert materials. Unless the mixtures are inherently safe, precautionary measures to prevent explosions are essential, and such precautionary measures can significantly increase the cost of the airstream process. Under essentially no operating conditions may there be fluctuations in the concentration, as such fluctuations could form currents or even dust deposits which contain hazardous carbon concentrations.
In two-component mixtures, on the basis of reactive calcium compounds and powdered activated cokes, the granular structure of the individual components is so different that disruptive fluctuations in concentration are essentially unavoidable. With activated cokes, depending on the variety used, the maximum grain size which characterizes the granular structure can be between about 200 xcexcm and about 500 xcexcm, and with calcium hydroxide as the reactive calcium compound, the maximum grain size can be up to approximately 30 xcexcm.
If such mixtures of different granular structure, e.g. mixtures of calcium hydroxide and activated coke, are used in the airstream process in exhaust gas cleaning systems, local fluctuations in concentration can occur in the flow field at points where there are centrifugal forces, such as in elbows. The larger particles are then enriched in the coarse grain fraction, i.e. there are higher carbon concentrations there.
It is essentially impossible to prevent small amounts of the coarse grain fraction which have an enriched carbon concentration from precipitating out of the flow field, and in particular at points through which there is not a strong flow. Laboratory tests with mixtures of calcium hydroxide and activated coke in laboratory wind sifters have shown that even with initial mixtures which have carbon concentrations of less than 30%, carbon enrichment can occur. Such carbon enrichments are generally unacceptable from a safety point of view.
It has been determined that this problem can essentially not be solved by the conventional grinding of the activated coke, in which the amount of coarse grain is limited so that even the coarsest particles can become airborne under the conditions of the airstream process (where the gas velocities are as low as 6 m/s).
Attempts to match or adjust the grain size distribution of the powdered activated cokes to the calcium hydroxide, so that both components have essentially the same distribution of the speed of vertical descent or drift, have shown that, of course, with sufficiently fine grinding, there are essentially no further fluctuations in the concentration. But, that would require that the activated carbons be ground down to approximately the grain size of the calcium hydroxide, i.e. to  less than 50 xcexcm. For technical reasons, grinding down activated carbons to match the grain size of calcium hydroxide would typically be difficult and time-consuming, as well as economically unacceptable.
Alternatively, if an attempt were made to agglomerate finely-divided calcium hydroxide to the point where the distribution of its speed of vertical descent or drift matched the distribution of the speed of vertical descent or drift of the activated coke, the effectiveness of the mixture for the separation of acid pollutant gases would be significantly reduced, so that such a solution also does not seem logical.
The object of the present invention is therefore to prepare a mixture which contains powdered, activated coke, in which carbon is present in a concentration of up to 30% by weight and which, when used in essentially all fractions and under essentially all operating conditions in the airstream process, is not subject to any fluctuations in concentration, as such fluctuations, at oxygen concentrations of up to 21% by volume, would cause the formation of fractions which could cause dust explosions.
The present invention relates to a powdered means for the cleaning of exhaust gases, e.g. gases exhausted from industrial smokestacks (smoke and chimney gases), employing the principle of dry adsorption. The powdered means generally includes a mixture of activated cokes with inert materials, which inert materials do not have any dust explosiveness.
The present invention teaches that this object can be achieved if, in the mixture, at least one inert component is matched in terms of its grain size distribution to the grain size distribution of the activated coke components, so that in a gas medium at rest, the speed of vertical descent of the coarse grain of the inert components is equal to or greater than the speed of vertical descent of the coarse grain of the activated coke components.
Coarse grain, in accordance with at least one embodiment of the present invention can be defined as the particle size d97, the cumulative value of which equals 97% of the particle size distribution (i.e. 97% of all the particles in the powder are smaller than d97).
With an accuracy which is acceptable in practical terms, equal speeds of vertical descent in the gas medium are achieved when the d97 values of the particle size distributions of the activated coke and of the inert material are in inverse proportion to the roots of the particle densities of activated coke and inert components (particle densities as defined in DIN 66160 (Deutsche Industrie-Norm or German Industrial Standards located at Deutsche Institut Fur Normung e.V., Sprachendienst, Post Fach 11 07, D-1000, Berlin 30, Federal Republic of Germany)).
In accordance with one embodiment of the present invention, equal speeds of vertical descent in the gas medium can possibly be achieved when the d97 values of the particle size distributions of the activated coke and of the inert material are in inverse proportion to the square roots of the particle densities of activated coke and inert components. In accordance with an alternative embodiment of the present invention, equal speeds of vertical descent in the gas medium can possibly be achieved when the d97 values of the particle size distributions of the activated coke and of the inert material are in inverse proportion to the cube roots of the particle densities of activated coke and inert components.
A preferred mixture is one in which the slope or gradient of the grain size distribution line of at least one inert component with matching grain size (i.e. the inert component having a grain size matching the grain size of the activated coke) is equal to or greater than the slope of the grain size distribution line of the activated coke component (See DIN 66145 for a description of grain size distributions by RRSB (Rosin-Rammler-Sperling-Bennett lines). Further explanations of grain size distributions by RRSB can be found in the Journal of the Institute of Fuel, volume 7, pages 29, in an article by Rosin and Rammler, and in volume 10, page 22, in an article by Bennett. These articles are hereby incorporated by reference herein.
In view of the above requirements, it has suprisingly been determined that powdered limestone can be used as the inert material. It is generally known that limestone is essentially calcium carbonate with a possible magnesium carbonate content.
Reactive calcium compounds with non-matching grain sizes can be used as additional inert materials. Calcium hydroxide is particularly preferred for such a purpose.
It is within the scope of the present invention that other inert materials could be used, in certain embodiments of the present invention, such as pulverized granite, slate, or quartz. Further, it is also conceivable that other types of reactive calcium compounds could be used in accordance with certain embodiments of the present invention, in addition to calcium hydroxide, such as calcium silicates, calcium alkalis, and calcium reactive silicas.
The powder mixture of the present invention has been found to be useful in essentially all exhaust gas cleaning systems having separately operated, upstream or downstream adsorption stages which employ the airstream principle.
If several inert components are used to achieve a properly matching distribution of the speed of vertical descent, it is of particular advantage if safe carbonaceous mixtures for the airstream process can be manufactured which contain as little additional inert material as possible. Likewise, it is possible to use, as the inert material, one or more exhaust gas cleaning reagents, if the reagents act as inert material with respect to the explosiveness of powdered activated coke, and if the reagents are present in the desired grain size distribution. One preferred inert material which has been found to satisfy the above requirements is powdered limestone, which is available economically and in many finenesses of grind. Powdered limestone is not a pollutant and is in no way toxic.
Additional advantages of the preferred mixtures manufactured in accordance with the present invention, as compared to the mixtures of the past, are that in mixtures of activated coke which use basic exhaust gas cleaning reagents as the inert material, it becomes possible to manufacture inherently safe mixtures with the smallest possible percentage of inert material which is matched to the grain size distribution of the activated coke and the percentage of carbon. In mixtures of activated cokes and an inert material which does not react with the exhaust gas, or reacts with it only to a limited extent, safe mixtures can be manufactured which contain carbon concentrations in percentages of up to 30%. The limitation of the percentages of inert components in the reagent preferably minimizes the amount which must be used and therefore can offer advantages in terms of cost.
In processes which employ the wet washing or scrubbing principle, the pre-cleaned exhaust gas can typically contain either no acid pollutant components or only very small concentrations of acid pollutant components. However, the pre-cleaned exhaust gas will still typically contain organic compounds and heavy metals.
In an upstream or downstream sorption stage, that is, upstream or downstream of the precleaning stage, one exhaust gas cleaning means in accordance with the present invention for cleaning exhaust gas can preferably have the following composition (parts by weight in %):
The principal ingredient in such a mixture can be an economical inert material, the speed of vertical descent of which material preferably matches the coarse activated coke breeze. If, under extreme circumstances deposits do occur in the exhaust gas cleaning system, there would typically also be an inert material content in the deposit, which inert material can then prevent undesirable and hazardous carbon concentrations in these deposits. The highly-reactive calcium hydroxide component in this mixture preferably makes possible a residual separation of acid exhaust gas components, if necessary.
This example of a mixture represents a solution for gas scrubbing which has been found to be both economical and optimal in terms of safety with regard to the relevant pollutant components of the exhaust gas.
In the process which employs the principle of spray sorption, the pre-cleaned exhaust gas, depending on the type of operation, can contain significant concentrations of acid pollutant components, as well as organic compounds and heavy metals.
In this case, the exhaust gas cleaning means in accordance with the invention can have the following composition in a downstream exhaust gas cleaning (parts by weight in %):
If such a mixture is used, in spite of the high reactivity of the calcium hydroxide additive with respect to acid pollutant components, minimal to no concentration fluctuations have been found to occur in any deposits which may occur, even in extreme cases, which fluctuations can lead to hazardous carbon concentrations.
The mixture can be used in spray sorption systems and also as a milk-of-lime suspension (i.e. a suspension of the mixture in water) in single-stage operations.
This example of a mixture represents a solution which is both economical and optimal from a safety point of view for spray sorptions of downstream exhaust gas cleaning systems with regard to the relative polluting components of the exhaust gas.
In processes which employ the dry sorption principle, depending on the type of operation, the pre-cleaned exhaust gas typically still contains significant concentrations of acid pollutant components as well as organic compounds and heavy metals.
In this case, the exhaust gas cleaning means claimed by the invention can have the following composition (parts by weight in %):
The use of such a mixture can essentially provide an effective adsorption of acid pollutant components of the exhaust gas. Such a mixture can also essentially provide that, in the event of unfavorable conditions in the exhaust gas cleaning system, no deposits of coarse particles with unacceptably high carbon concentrations can occur.
The above examples of various compositions of mixtures in accordance with the present invention show that it is possible to solve the problems of unacceptable fluctuations in carbon concentrations in deposited material both in separately operated adsorption stages, and in upstream or downstream adsorption stages, for the relevant pollutant components.
When the word xe2x80x9cinventionxe2x80x9d is used in this specification, the word xe2x80x9cinventionxe2x80x9d includes xe2x80x9cinventionsxe2x80x9d, that is, the plural of xe2x80x9cinventionxe2x80x9d. By stating xe2x80x9cinventionxe2x80x9d, the Applicants do not in any way admit that the present application does not include more than one patentably and non-obviously distinct invention, and maintains that this application may include more than one patentably and non-obviously distinct invention. The Applicants hereby assert that the disclosure of this application may include more than one invention, and in the event that there is more than one invention, that these inventions may be patentable and non-obvious one with respect to the other.
One aspect of the invention resides broadly in a method of reducing explosions during the cleaning of exhaust gas, the method comprising the steps of: providing activated carbon, tbe activated carbon having a first grain size distribution; providing at least one inert material, the at least one inert material having a second grain size distribution, the second grain size distribution being substantially the same as the first grain size distribution; preparing a mixture of the activated carbon and the at least one inert material; introducing the mixture into a flow of exhaust gas; and cleaning the exhaust gas by adsorption of portions of the exhaust gas by the activated carbon.
Another aspect of the invention resides broadly in powdered means for cleaning exhaust gases using the principle of dry adsorption which consists of a mixture of activated cokes with inert materials which have no dust explosiveness, characterized by the fact that in the mixture, at least one of the inert material components is matched in terms of its grain size distribution to the grain size distribution of the activated coke component so that in a gas medium at rest, the speed of vertical descent of the coarse grain of the inert material component is equal to or greater than the speed of vertical descent of the coarse grain of the activated coke component.